The present invention relates to ultrasonic tissue imaging techniques and, in particular, it concerns a method and apparatus for the ultrasonic evaluation of bone tissue.
It is known that ultrasonography is often used for diagnostic tissue imaging in human beings. As soft or fluid filled tissues possess favorable acoustic properties, ultrasonography is able to provide excellent imagine of these tissues. The ultrasonic evaluation of bone tissue, however (for example, for estimating the degree of osteoporosis, and thus bone fracture risk) is problematic, due to the difficulty in achieving adequate ultrasound penetration in complex solid biological structures Such as bone. To date, therefore, the reliable ultrasonic imaging of bone structure and density and has not been possible.
Ultrasonic evaluation of bone tissue, as with any biological tissue, is achieved by transmitting an ultrasonic pulse or pulses into the bone tissue, and then analyzing the acoustic qualities of the received reflected ultrasonic signals. Properties of bone tissue can then be determined by analyzing the amplitude and/or travel time of the received signals. The amplitude of the received pulses, which indicates the degree of attenuation of the transmitted ultrasound signals, correlates with bone mineral density. The travel time of the signal transmitted through the bone tissue is used for calculating the velocity of the ultrasound signal within the bone tissue, the so-called "speed of sound" (SOS), which also correlates with the degree of osteoporosis and/or risk of bone fracture.
Several techniques for the ultrasonic evaluation of bone tissue are known in the art. FIG. 1 depicts a conventional ultrasonic apparatus for evaluation of bone tissue, generally designated 10. Ultrasonic apparatus 10 for the evaluation of bone tissue includes an ultrasonic probe 12 for transmitting ultrasonic pulses towards a bone 14 via soft tissue 16, and for receiving signals reflected from or transmitted through, bone 14. Ultrasonic probe 12 is typically a hand-held implement for manipulation by an operator. The operator grips ultrasonic probe 12 and applies it to soft tissue 16. As the surface of bone 14 is inaccessible for direct coupling with ultrasonic probe 12, the operator is required to adjust the position and apposition of ultrasonic probe 12 on soft tissue 16, in order to optimize the transmission into, and reception from, bone 14 of ultrasound signals. When ultrasonic probe 12 is optimally oriented, the amplitude of the received signals is maximal while the time of flight is minimal.
Ultrasonic apparatus 10 for evaluation of bone tissue further includes a digital computing device 18 for analyzing the received ultrasound signal and generating an image of bone 14 from the measured amplitude and/or time delay of the received signal. Ultrasonic apparatus 10 for evaluation of bone tissue also includes a display 20 for displaying the image generated by computing device 18.
Turning now to FIG. 2, a part of ultrasonic apparatus 10 is depicted, including ultrasonic probe 12. As the internal structure of bone 14 is inhomogeneous, the ultrasound signal received by probe 12 typically has a low signal to noise ratio. As such, the through transmission technique is typically employed, in which one transducer (that is, a scanning crystal) transmits signals while a second transducer receives the signals after they have traveled through the substance under investigation.
Ultrasonic probe 12 typically includes two resonant scanning crystals 22 and 24, which work at a fixed frequency, and which are connected to digital computing device 18. Scanning crystal 22 is operative to transmit ultrasonic pulses toward bone 14 via soft tissue 16, while scanning crystal 24 is operative to receive ultrasonic signals which have passed through, or been reflected by, bone 14 and soft tissue 16. Each of scanning crystals 22 and 24 have inclined delay lines 26 and 28 respectively. In other words, the part of the transducer in front of the scanning crystal, through which the longitudinal waves generated by the scanning crystal pass prior to entering the tissue to which the transducer has been applied, is inclined at an acute angle to the surface of that tissue. The velocity of ultrasound within delay lines 26 and 28 is approximately equal to the velocity of ultrasound in soft tissue 16. Delay line 26 typically directs scanning crystal 22 at an angle .alpha. with regard to the surface of soft tissue 16, so as to cause propagation of longitudinal leaky waves along the surface of bone 14. Delay line 28 directs scanning crystal 24 by the same angle .alpha. with regard to the surface of soft tissue 16, so as to facilitate optimal reception of the ultrasound signal passed along bone 14.
The net travel time for ultrasound signals that have passed through bone 14 is described by the formula: EQU T.sub.14 =T.sub..SIGMA. -T.sub.26 -T.sub.28 -T.sub.16,
where T.sub.14 is the net travel time for a signal passed through bone 14; T.sub..SIGMA. is the time delay between transmission of an ultrasonic pulse by scanning crystal 22 and reception of the pulse by scanning crystal 24; T.sub.26 and T.sub.28 are the propagation times for ultrasonic pulses in delay lines 26 and 28 respectively; and T.sub.16 is the propagation time for ultrasonic pulses in soft tissue 16.
Two auxiliary crystals 30 and 32 are located in ultrasonic probe 12, and are connected to digital computing device 18. Auxiliary crystals 30 and 32 are typically used to determine the propagation time for ultrasonic pulses in soft tissue 16. This is achieved by crystal 30 transmitting an ultrasonic pulse into soft tissue 16 while crystal 32 receives the reflected echo pulse from the surface of bone 14. The measured delay between transmission and reception of this echo pulse determines the value of T.sub.16.
The velocity of ultrasound (SOS) in bone 14 is described by the formula: ##EQU1##
Per the following reason:
It is well known that ##EQU2## PA1 SOS is defined as velocity; BTD is defined as distance and T is defined as time.
where BTD is the bone travel distance, which is determined by the distance between scanning crystals 22 and 24 and the value of angle .alpha..
Ultrasonic travel time and/or amplitude measurements for an ultrasonic pulse which has passed through bone 14 are heavily influenced by the proficiency with which the operator applies ultrasonic probe 12 to soft tissue 16. Several techniques for maximizing operator proficiency have been described in the art. A typical technique is illustrated in FIG. 3, in which a part of ultrasonic apparatus 10 is depicted, including ultrasonic probe 12. As shown in the figure, additional auxiliary crystals 34 and 36 are located within probe 12, and are connected to digital computing device 18. Crystal 34 is operative to transmit ultrasonic pulses into soft tissue 16, while crystal 36 is operative to receive the reflected echo pulse from the surface of bone 14. The measured delay between transmission and reception of said echo pulse is T.sub.16a. When I.sub.16 =T.sub.16a, probe 12 is oriented in such a way that the BTD will be the shortest possible for that probe. A smaller value for BTD minimizes the impact of inevitable inaccuracies in the calculation of SOS. Thus, when digital computing device 18 determines that T.sub.16 =T.sub.16a, probe 12 is deemed to be oriented appropriately with regard to soft tissue 16, and the received echo signals are analyzed so as to image bone 14. When the condition T.sub.16 T.sub.16a is not met, received ultrasound signals are ignored by digital computing device 18.
In an alternative method for minimizing operator unreliability, the operator applies ultrasonic probe 12 to a reference block made from material with known acoustical properties prior to applying probe 12 to soft tissue 16 and bone 14. The operator can then compare the actual images obtained from bone 14 with the "optimal" images obtained from the reference block, and continues to adjust the orientation of probe 12 until such time as the current image approximates the "optimal images."
The above-described methods for ultrasonic imaging of bone, however suffer from several deficiencies:
1. It is common experience that the repeatability and precision of travel time and amplitude measurements for signals passed through bone 14 is low, even when optimal orientation of ultrasonic probe 12 with respect to bone 14 is achieved. Furthermore, as the exact propagation times T.sub.26 and T.sub.28 of ultrasonic signals in delay lines 26 and 28 are unknown, calculated values for ultrasound velocity (SOS) are unreliable. PA0 2. The methods used for optimizing the orientation of probe 12 with regard to bone 14 do not relate to the signal actually received from bone 14, but rather, infer an optimal bone-probe orientation from signals received from other materials (either soft tissue 16 or a reference block). PA0 3. As the dense cortex of bone 14 distorts transmitted signals, current fixed-frequency ultrasonic bone imaging techniques allow only for an integral evaluation of the surface of bone 14, but not for the imaging of the internal structure of bone 14 (for example, so as to reveal local inhomogeneities and fractures). Furthermore, as current techniques utilize ultrasonic pulses of a single, fixed, frequency- and measure only amplitude or travel time changes in the received signal-additional ultrasonic phenomena, such as possible changes in the frequency spectrum of the transmitted pulse induced by the internal structure of bone, are not evaluated. Such phenomena, however, may reveal information about the internal structure of bone, which cannot be inferred from single parameter measurements (such as amplitude or travel time).
There is therefore a need for, and it would be highly advantageous to have a method and device for achieving ultrasonic imaging of bone tissue which would allots for the precise and easily repeatable measurement of ultrasonic travel time and signal amplitude, the imaging of the internal structure of bone tissue, and the optimization of probe orientation by directly utilizing the imaging signals received from the bone.